Lamivudine is known by the chemical name (2R-cis)-4-Amino-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]-2(1H)-pyrimidinone. Lamivudine is represented by the following structure.

Lamivudine is a reverse transcriptase inhibitor used in the treatment of HIV infection alone or in combination with other class of Anti HIV drugs.
Lamivudine may be prepared using the procedures described in WO 91/17159.
It has been reported in literature [J. Pharm. Sci., 1996, 85, 193-199; U.S. Pat. No. 5,905,082] that lamivudine exists in two polymorphs known as polymorphic Form I and Form II. Form I of lamivudine is a needle shaped crystals. Form II of lamivudine is a bipyramidyl crystals. Lamivudine in the form of bipyramidyl crystals has a melting point of 177-178° C. and a melting point of 124-127° C. when in the form of needle shaped crystals.
WO 03027106 A1 discloses a process for the preparation of lamivudine polymorph Form II. However, lamivudine in this form I is not suitable for pharmaceutical formulations in the form of solid dosage forms because of their physical properties, such as poor flow characteristics. Form I crystals are a less stable polymorphic form and certain pharmaceutical unit operations such as milling may cause conversion of form I to form II which is an undesirable characteristic for manufacture of solid dosage forms.
WO 2007/119248 discloses crystalline hemihydrate (Form III) of lamivudine. Crystalline form I have inferior flow property and also lower bulk density, which create problem in handling the product during formulation.
It has been found that lamivudine polymorphs form I which is stable during pharmaceutical operations such as milling, can be prepared by following certain specific procedure.
Thus, the object of the invention is to prepare stable lamivudine polymorph form I suitable for pharmaceutical preparation.